A bandgap reference circuit generates an output voltage that is substantially independent of a temperature variation, and thus has been used in integrated circuits to supply one or more of stable operating voltages. In order to supply the operating voltages, a conventional bandgap reference circuit adds a proportional-to-absolute-temperature (PTAT) voltage to a complementary-to-absolute-temperature (CTAT) voltage. For example, a CMOS bandgap reference circuit includes a complementary metal-oxide semiconductor (CMOS) operational amplifier (op-amp) to generate the PTAT voltage. However, an offset of the CMOS op-amp may result in an output voltage with significant temperature drift.